U.S. Patent Documents
Not Applicable.
Not Applicable.
1. Field of Invention
The present invention relates to electrical stepping motors, and more particularly, to micro-miniature stepping motors used to drive the gear works for turning the time-indicating hands in quartz analog timepieces.
2. Description of Prior Art
A stepping motor is one that rotates by way of short, essentially uniform angular movements rather than continuously. An important application of stepping motors is in electronic watches having an analog display formed by rotating hands. In a watch of this type, low-frequency timing pulses, derived by frequency division from a high frequency crystal-controlled time base, serve to actuate a stepping motor which drives the gear works rotating the hands of the watch.
Various types of stepping motors are known which make use of a rotor incorporating a drive member such as a gear pinion, a permanent magnet, usually having two poles, and a pinion shaft with journals rotatably mounted in the watch movement. The permanent magnet is often in the shape of a ring with a central hole through it for the pinion shaft. A typical shape would be a toroid measuring about 1.50 mm O.D.xc3x970.35 mm I.D.xc3x970.50 mm thick.
In the context of an electronic watch, the stepping motor must not only be in micro-miniature form to minimize space requirements, in order to make same particularly suitable for use in a small sized wristwatch, but its power consumption must be extremely low. In order to energize the watch with a miniature power cell which will last at least a year, the allowable power consumption is usually less than 8 microwatts. Another desirable characteristic of a stepping motor rotor is that it have a low moment of inertia about its axis of rotation.
Permanent magnets used for watch rotors, typically anisotropic rare earths such as samarium-cobalt, are extremely brittle and difficult to machine. Because of the small size of the rotor, close tolerances are required in the stepping motor. Various improvements have been suggested to reduce breakage of the magnets and to reduce the cost of manufacturing a stepping motor rotor.
The possibility of breakage increases when designs require a press fit of the magnet material to the rotor pinion shaft, or when compressive forces on the outer diameter are necessary to hold it in place on the rotor. Protective bushings have been used to prevent breakage but, since the moment of inertia varies as the square of the radius of rotation, the use of protective bushings between the inner diameter of the magnet and the rotor shaft or the use of metallic shells encasing the outer diameter of the magnet is to be avoided.
Examples of prior art rotor assemblies with plastic internal bushings disposed between the shaft and the magnet are shown in Onda, U.S. Pat. No. 4,206,379; Bannon, U.S. Pat. No. 3,953,752; Kooyman, U.S. Pat. No. 2,488,729 and Migeon, U.S. Pat. No. 4,340,560.
An example of a stepping motor rotor with permanent magnet clamped axially between metal bushings press fit with an interference fit to the rotor shaft is seen in Kusayama, U.S. Pat. No. 4,035,677.
Constructions where the inner hole diameter of the permanent magnet is finished and pressed directly onto the rotor shaft are shown in Ono, U.S. Pat. No. 3,943,698 which uses a reinforcing plate of stainless steel or other non-magnetizable material bonded to the magnet to prevent cracking the brittle material.
A rotor assembly which employs a radial wall and outer protective metal sheath into which the magnet is press fit on its outer diameter is disclosed in Tanai, U.S. Pat. No. 4,095,129. Although this permits forming the inner diameter of the rare earth magnet by a rough drilling process, the outer metallic protecting sheath both increases the moment of inertia and offer the possibility of breakage by compressive stresses due to the press fit on the outer diameter of the magnet.
The magnet rotor must hold its strength under adverse conditions, otherwise it will change in its performance. Hence, high coercivity and temperature stability are essential. Finally, in order to reduce the magnet volume and weight, it is important that the magnet have a high energy product (BHmax). Typically, the specification for the energy product of rare earth watch rotors is at least 22 MGOe (170 kJ/m3).
Only the rare earth permanent magnet materials samarium-cobalt, Sm2Co17, and neodymium-iron-boron, Nd2Fe14B, usually shortened to just NdFeB, have energy products greater than 22 MGOe (170 kJ/m3). Sm2Co17 has a high energy product at about 30 MGOe (238 kJ/m3) and high coercivity at about 10 KOe. Stable at temperatures up to 350xc2x0 C. it is also very resistant to corrosion. Its disadvantages are its high cost and difficulty to machine.
NdFeB has an exceptionally high energy product at about 40 MGOe (318 kJ/m3) and exceptionally high coercivity at about 15 KOe. Relatively easy to machine and relatively inexpensive, the major disadvantage of NdFeB magnets is their poor corrosion resistance and instability above 150xc2x0 C. These shortcomings severely limit their application.
Studies aimed at improving the corrosion resistance of NdFeB magnets have mostly emphasized protective coatings or the addition of alloying elements, however, the effectiveness of coatings is not always assured and alloying to increase the corrosion resistance usually reduces magnetic properties. While new corrosion resistant NdFeB alloys may change the situation in future, the current use of Sm2Co17 for quality watch rotors is almost universal.
Sm2Co17 magnets are anisotropic and must be magnetized in the orientation direction. Hence, the provision of a prealloyed powder is a prerequisite. The process starts by vacuum induction melting a carefully optimized blend of alloying ingredients and casting a Sm2Co17 ingot. The ingot is then crushed under protective atmosphere to a coarse, typically minus 50 mesh (297 micron) prealloyed powder. The resulting powder is further coarse ground, usually by autogenous grinding under hydrogen atmosphere in a heated cylindrical mill (hydrogen decrepitation method). Finally, the powder is jet milled under high pressure (about 120 psi) N2 to a critical size depending on the size of individual crystallites. Following screening to remove undesirable undersize and oversize particles the highly pyrophoric powder is stored under argon atmosphere until ready for pressing.
Pressing starts by blending a powder mixture based on chromatography results. Depending on the type of magnet being produced, the powder is either isostatically pressed into a blockxe2x80x94the method used in the fabrication of watch rotorsxe2x80x94or die pressed to a particular component shape. In either case the operation is conducted in a pulsed magnetic field (typically 10 kOe). The effectiveness of the pulses in magnetically aligning the crystallites diminishes as the powder is being compacted. During the latter part of the pressing step, stresses introduced as a result of plastic deformation as well as density gradients may lead to a less than perfect grain alignment.
Although the magnetic properties of isostatically pressed parts are usually higher than those of pressed parts, the uniformity of the magnetic characteristics of the former is usually lower than that of the latter. After pressing, the block or shapes are demagnetized with a decaying 60 Hz field.
Sintering is performed in high vacuum. The partial formation of liquid phase during sintering affects the angularity tolerance of the magnetization. This, combined with imperfect grain alignment results in difficulties to achieve the theoretical maximum energy product of Sm2CO17.
Sintering is followed by quenching and aging. The intrinsic coercivity is defined during the quenching step. The presence of large amounts of non-magnetic secondary phase adversely affects the energy product. The sintered ingots are diamond-sawed to the required dimensions and ground to the required tolerance. The blocks are normally pulse magnetized at 40-50 kOe before shipping unless magnetization of the final machined productxe2x80x94as in the case of watch rotors and disk drive magnetsxe2x80x94is performed at the end users"" facilities in which case they are shipped in the unmagnetized state.
Magnet blocks destined for watch rotors typically measure about 200 mmxc3x9750 mmxc3x9738 mm. First the ingot is diamond-sawed into roughly 0.5 mm thick slabs. Each slab is then cut into roughly 1.5 mm wide strips and each strip is further cut into roughly 1.5 mm square chips. The chips are then centerless-ground to the precise outside diameter. Next, each separate round has a hole drilled in it using an Excimer laser. The rough edges of the laser-drilled hole must be ground to the precise inside diameter, an operation requiring stringing up the toroids on a tungsten wire lubricated with diamond paste, and jigging the wire back and forth through the stacked parts rigidly held in place in a bath of frozen low temperature fusible metal alloy. Finally, the rotors are spot welded onto the pinion shaft using a laser beam.
Considering the rotor dimensions given above, a typical sawing kerf width of 0.5 mm and 100% yield, it is easily determined that, out of a 200 mmxc3x9750 mmxc3x9738 mm magnet ingot, only 20% of the material is effectively turned into rotors. Practically, due to poor grinding yield, actual material utilization is rarely higher than 10-14%
As can be inferred from above explanations, the prior art process of making Sm2Co17 watch rotors is complex, time-consuming, labor-intensive, and wasteful in terms of material and energy utilization.
In accordance with the present invention the labor-intensive and inefficient techniques of the prior art are substantially overcome by forming rare earth watch rotors from a mixture of prealloyed rare earth magnetic particles and a thermoplastic binder. Green watch rotors are produced by either casting the mixture into a tape in a magnetic field, followed by blanking of the rotors or, alternatively, by injection molding the mixture in a mold cavity placed in a magnetic field. Following extraction of the binder the green parts are sintered to net shape. Improved magnetic properties, smaller dimensions, better tolerances and 100% material utilization are thus achieved.
It is the primary object of this invention to provide an economic, simple, energy and material efficient process to mass-produce rare earth magnet rotors for quartz watches.
An additional object of this invention is to provide a method to fabricate rare earth watch rotors that are smaller in size and with greater dimensional precision than in the prior art.
Fine prealloyed samarium cobalt powder being extremely reactive and pyrophoric, sintering of the prior art ingots is necessarily performed as soon as possible after the cast ingot has been crushed and milled into a powder. As a result, the vacuum induction melting, casting, comminution, pressing and sintering of the samarium cobalt alloy are, of necessity, carried out in the same facilities. This situation has lead to a virtual monopoly over rare earth magnet manufacturing by a handful of manufacturers.
It is an object of this invention to reduce the end user""s dependence on suppliers of sintered samarium-cobalt and other permanent magnet ingots. Samarium in solid or powder form can be easily obtained from suppliers who are not magnet producers themselves. Likewise cobalt in granules, flakes or powder can be easily procured from cobalt producers. Sourcing raw materials from non magnet producers inherently entails substantial cost reductions. Samarium-cobalt alloys can be cast by any foundry equipped with a vacuum induction furnace. Likewise crushing and rough grinding of the cast ingot does not represent any problem since it is only the fine jet milled powder that becomes pyrophoric. Hence this invention has the potential to dramatically reduce the raw material cost for rare earth magnets.
Conventional isostatic pressing of samarium-cobalt ingots in a magnetic field is done with bulky isostatic presses which represent a sizeable investment. This invention does not require such investment. Furthermore, as no pressing is involved there are no stresses nor density gradients when sintering is initiated.
This invention bypasses the entire samarium-cobalt powder pressing, ingot sintering and machining process, replacing it with a simple, economical, zero-waste and environmentally clean process eminently suited to automation.
Finally this invention allows for higher energy products to be realized than by using the prior art methodology as magnetic alignment of the particulates is optimized as it takes place while the binder is in a fluid condition and the magnetic particles have more freedom of movement than when they are isostatically pressed together as is the prior art.
Not applicable.